How do the performance and cost of DRAM compare with NAND flash?
DRAM handily outperforms NAND flash in terms of performance, especially when comparing write operations. This is because NAND flash doesn't overwrite existing data at the byte level. It has to write data in full block increments, which requires that an entire block be erased before a new block of data can be written. The cost of DRAM is significantly more expensive and less dense. This means you have to buy many more DRAM modules to reach the same level of capacity as flash.
Other than the cost of DRAM, it has another distinct disadvantage: It's volatile. If it loses power it will also lose the data it stores. However, there are technologies out (and more coming on the horizon) that address this volatility. A particularly interesting one is NVDIMM (nonvolatile dual inline memory module), which combines flash and DRAM on the same memory module. If there is a power failure, a capacitor on the module will allow the data in DRAM to be copied to the flash portion of the module before the DRAM fails. When power is returned, the data is copied back to DRAM. This provides a pause/resume feature for storage systems and servers.
While DRAM may never be used en masse, combining it with a capacitor and flash could allow it to be a very effective write cache tier or a highly reliable server memory solution that can survive power loss.
This was first published in January 2014